Manipulating the electron dynamics in the non-sequential double ionization process of Ar atoms by an orthogonal two-color laser field

Electron dynamics during non-sequential double ionization(NSDI) is one of the most attractive areas of research in the field of laser–atom or laser–molecule interaction. Based on the classic two-dimensional model, we study the process of NSDI of argon atoms driven by a few-cycle orthogonal two-color laser field composed of 800 nm and 400 nm laser pulses. By changing the relative phase of the two laser pulses, a localized enhancement of NSDI yield is observed at 0.5πand 1.5π, which could be attributed to a rapid and substantial increase in the number of electrons returning to the parent ion within extremely short time intervals at these specific phases. Through the analysis of the electron–electron momentum correlations within different time windows of NSDI events and the angular distributions of emitted electrons in different channels, we observe a more pronounced electron–electron correlation phenomenon in the recollision-induced ionization(RII) channel. This is attributed to the shorter delay time in the RII channel.

Ultrafast dynamics of femtosecond laser-induced high spatial frequency periodic structures on silicon surfaces

Femtosecond laser-induced periodic surface structures(LIPSS)have been extensively studied over the past few decades.In particular,the period and groove width of high-spatial-frequency LIPSS(HSFL)is much smaller than the diffraction limit,making it a useful method for efficient nanomanufacturing.However,compared with the low-spatial-frequency LIPSS(LSFL),the structure size of the HSFL is smaller,and it is more easily submerged.Therefore,the formation mechanism of HSFL is complex and has always been a research hotspot in this field.In this study,regular LSFL with a period of 760 nm was fabricated in advance on a silicon surface with two-beam interference using an 800 nm,50 fs femtosecond laser.The ultrafast dynamics of HSFL formation on the silicon surface of prefabricated LSFL under single femtosecond laser pulse irradiation were observed and analyzed for the first time using collinear pump-probe imaging method.In general,the evolution of the surface structure undergoes five sequential stages:the LSFL begins to split,becomes uniform HSFL,degenerates into an irregular LSFL,undergoes secondary splitting into a weakly uniform HSFL,and evolves into an irregular LSFL or is submerged.The results indicate that the local enhancement of the submerged nanocavity,or the nanoplasma,in the prefabricated LSFL ridge led to the splitting of the LSFL,and the thermodynamic effect drove the homogenization of the splitting LSFL,which evolved into HSFL.

Periodic transparent nanowires in ITO film fabricated via femtosecond laser direct writing

This paper reports the fabrication of regular large-area laser-induced periodic surface structures(LIPSSs)in indium tin oxide(ITO)films via femtosecond laser direct writing focused by a cylindrical lens.The regular LIPSSs exhibited good properties as nanowires,with a resistivity almost equal to that of the initial ITO film.By changing the laser fluence,the nanowire resistances could be tuned from 15 to 73 kΩ/mm with a consistency of±10%.Furthermore,the average transmittance of the ITO films with regular LIPSSs in the range of 1200-2000 nm was improved from 21%to 60%.The regular LIPSS is promising for transparent electrodes of nano-optoelectronic devices-particularly in the near-infrared band.

Large-area straight,regular periodic surface structures produced on fused silica by the interference of two femtosecond laser beams through cylindrical lens

Inhomogeneity and low efficiency are two important factors that limit the application of laser-induced periodic surface structures(LIPSSs),especially on glass surfaces.In this study,two-beam interference(TBI)of femtosecond lasers was used to produce large-area straight LIPSSs on fused silica using cylindrical lenses.Compared with those produced us-ing a single circular or cylindrical lens,the LIPSSs produced by TBI are much straighter and more regular.Depending on the laser fluence and scanning velocity,LIPSSs with grating-like or spaced LIPSSs are produced on the fused silica sur-face.Their structural colors are blue,green,and red,and only green and red,respectively.Grating-like LIPSS patterns oriented in different directions are obtained and exhibit bright and vivid colors,indicating potential applications in surface coloring and anti-counterfeiting logos.

Femtosecond laser-induced periodic structures:mechanisms, techniques, and applications

Over the past two decades,femtosecond laser-induced periodic structures(femtosecond-LIPSs)have become ubiquitous in a variety of materials,including metals,semiconductors,dielectrics,and polymers.Femtosecond-LIPSs have become a useful laser processing method,with broad prospects in adjusting material properties such as structural color,data storage,light absorption,and luminescence.This review discusses the formation mechanism of LIPSs,specifically the LIPS formation processes based on the pump-probe imaging method.The pulse shaping of a femtosecond laser in terms of the time/frequency,polarization,and spatial distribution is an efficient method for fabricating high-quality LIPSs.Various LIPS applications are also briefly introduced.The last part of this paper discusses the LIPS formation mechanism,as well as the high-efficiency and high-quality processing of LIPSs using shaped ultrafast lasers and their applications.

Ultrafast Modulation of the Molten Metal Surface Tension under Femtosecond Laser Irradiation

We predict ultrafast modulation of the pure molten metal surface stress fields under the irradiation of the single femtosecond laser pulse through the two-temperature model molecular-dynamics simulations. High-resolution and precision calculations are used to resolve the ultrafast laser-induced anisotropic relaxations of the pressure components on the time-scale comparable to the intrinsic liquid density relaxation time. The magnitudes of the dynamic surface tensions are found being modulated sharply within picoseconds after the irradiation, due to the development of the nanometer scale non-hydrostatic regime behind the exterior atomic layer of the liquid surfaces.The reported novel regulation mechanism of the liquid surface stress field and the dynamic surface tension hints at levitating the manipulation of liquid surfaces, such as ultrafast steering the surface directional transport and patterning.

Preparation of AgCl Nano-Crystal Embedded Tellurite Nonlinear Optical Glasses under Electric Field Accompanied Heat Treatment

The quantum effect of nano-crystals is an important factor to improve nonlinear optical performance of nanocrystal embedded glasses, while controlling the size distribution and content of nano-crystals in the glass accurately is a key to obtain good quality. The auxiliary direct current electric field, accompanied with heat treatment, was applied on AgCI containing niobic tellurite glass sheet. The nucleation and crystallization of the glass were well controlled under auxiliary electric field. It was found that the average size of AgCI nano-crystal particles in the glass is smaller than that under single heat treatment, and the content of nano- crystals is higher. Therefore the third-order nonlinear optical performance of the glass was increased a lot. The local-area distributed AgCl nano-crystal particles can also be embedded into a glass sheet by using locally applied electric field.

Selective Excitation of Two-pulse Femtosecond Coherent Anti-Stokes Raman Scattering in a Mixture

In this paper, we experimentally study the selective excitation of two-pulse femtosecond coherent anti-Stokes Raman scattering (CARS) in a mixture of dibromomethane (CH2Br2) and chloroform (CHCl3) by adaptive pulse shaping based on genetic algorithm. Second harmonic generation frequency-resolved optical gating (SHG-FROG) traces indicate that the spectral amplitude and phase of the optimal pulse are both modulated. Finally, we discuss the physical mechanism for the selective excitation of femtosecond CARS based on the retrieved information from SHG-FROG traces.

Faster structured illumination microscopy using complementary encoding-based compressive imaging

Structured illumination microscopy (SIM) has been widely applied to investigate intricate biological dynamics due to its outstanding super-resolution imaging speed. Incorporating compressive sensing into SIM brings the possibility to further improve the super-resolution imaging speed. Nevertheless, the recovery of the superresolution information from the compressed measurement remains challenging in experiments. Here, we report structured illumination microscopy with complementary encoding-based compressive imaging (CECI-SIM) to realize faster super-resolution imaging. Compared to the nine measurements to obtain a super-resolution image in a conventional SIM, CECI-SIM can achieve a super-resolution image by three measurements;therefore, a threefold improvement in the imaging speed can be achieved. This faster imaging ability in CECI-SIM is experimentally verified by observing tubulin and actin in mouse embryonic fibroblast cells. This work provides a feasible solution for high-speed super-resolution imaging, which would bring significant applications in biomedical research.

Temporal compressive super-resolution microscopy at frame rate of 1200 frames per second and spatial resolution of 100 nm

Various super-resolution microscopy techniques have been presented to explore fine structures of biological specimens.However,the super-resolution capability is often achieved at the expense of reducing imaging speed by either point scanning or multiframe computation.The contradiction between spatial resolution and imaging speed seriously hampers the observation of high-speed dynamics of fine structures.To overcome this contradiction,here we propose and demonstrate a temporal compressive super-resolution microscopy(TCSRM)technique.This technique is to merge an enhanced temporal compressive microscopy and a deep-learning-based super-resolution image reconstruction,where the enhanced temporal compressive microscopy is utilized to improve the imaging speed,and the deep-learning-based super-resolution image reconstruction is used to realize the resolution enhancement.The high-speed super-resolution imaging ability of TCSRM with a frame rate of 1200 frames per second(fps)and spatial resolution of 100 nm is experimentally demonstrated by capturing the flowing fluorescent beads in microfluidic chip.Given the outstanding imaging performance with high-speed super-resolution,TCSRM provides a desired tool for the studies of high-speed dynamical behaviors in fine structures,especially in the biomedical field.

单曝光偏振映射超快成像

单曝光超快光学成像可以捕捉发生在纳秒时间尺度内的瞬态场景,尤其是对于泵浦探测技术无能为力的不可逆或难以重复的场景更具优势.为了打破传统CCD或CMOS相机的数据读出速度限制,单曝光超快成像技术通常将时间信息转换到波长、空间或空间频域等来进行超快探测.本文作者开发了一种新型时间-偏振映射的超快成像技术(PUMP),首先利用石英旋光晶体产生具有时变偏振角的超短激光脉冲照明动态场景,然后通过偏振滤波阵列对不同时刻信息进行快照曝光,最后结合迭代去卷积算法,实现了将时域信息转换到偏振维度的超快成像.此外,作者利用该新型超快成像系统实时观测了氧化铟锡薄膜的超快激光烧蚀动力学过程,验证了其在超快探测中的出色性能.该技术的发展有望进一步推动超快光学成像在基础和应用科学研究领域的应用.

Extremely regular periodic surface structures in a large area efficiently induced on silicon by temporally shaped femtosecond laser

Femtosecond laser-induced periodic surface structures(LIPSS) have several applications in surface structuring and functionalization. Three major challenges exist in the fabrication of regular and uniform LIPSS: enhancing the periodic energy deposition, reducing the residual heat, and avoiding the deposited debris. Herein, we fabricate an extremely regular low-spatial-frequency LIPSS(LSFL) on a silicon surface by a temporally shaped femtosecond laser. Based on a 4 f configuration zero-dispersion pulse shaping system, a Fourier transform limit(FTL) pulse is shaped into a pulse train with varying intervals in the range of 0.25–16.2 ps using periodic π-phase step modulation. Under the irradiation of the shaped pulse with an interval of 16.2 ps, extremely regular LSFLs are efficiently fabricated on silicon. The scan velocity for fabricating regular LSFL is 2.3 times faster, while the LSFL depth is 2 times deeper, and the diffraction efficiency is 3 times higher than those of LSFL using the FTL pulse.The formation mechanisms of regular LSFL have been studied experimentally and theoretically. The results show that the temporally shaped pulse enhances the excitation of surface plasmon polaritons and the periodic energy deposition while reducing the residual thermal effects and avoiding the deposition of the ejected debris, eventually resulting in regular and deeper LSFL on the silicon surface.

Valence state manipulation of Sm3＋ ions via a phase-shaped femtosecond laser field

The ability to manipulate the valence state conversion of rare-earth ions is crucial for their applications in color displays, optoelectronic devices, laser sources, and optical memory. The conventional femtosecond laser pulse has been shown to be a well-established tool for realizing the valence state conversion of rare-earth ions, although the valence state conversion efficiency is relatively low. Here, we first propose a femtosecond laser pulse shaping technique for improving the valence state conversion efficiency of rare-earth ions. Our experimental results demonstrate that the photoreduction efficiency from Sm^(3+) to Sm^(2+) in Sm^(3+)-doped sodium aluminoborate glass using a π phase step modulation can be comparable to that using a transform-limited femtosecond laser field, while the peak laser intensity is decreased by about 63%, which is very beneficial for improving the valence state conversion efficiency under the laser-induced damage threshold of the glass sample. Furthermore, we also theoretically develop a(2 + 1)resonance-mediated three-photon absorption model to explain the modulation of the photoreduction efficiency from Sm^(3+)to Sm^(2+)under the π-shaped femtosecond laser field.

Single-shot compressed ultrafast photography: a review

Compressed ultrafast photography(CUP)is a burgeoning single-shot computational imaging technique that provides an imaging speed as high as 10 trillion frames per second and a sequence depth of up to a few hundred frames.This technique synergizes compressed sensing and the streak camera technique to capture nonrepeatable ultrafast transient events with a single shot.With recent unprecedented technical developments and extensions of this methodology,it has been widely used in ultrafast optical imaging and metrology,ultrafast electron diffraction and microscopy,and information security protection.We review the basic principles of CUP,its recent advances in data acquisition and image reconstruction,its fusions with other modalities,and its unique applications in multiple research fields.

High-fidelity image reconstruction for compressed ultrafast photography via an augmented-Lagrangian and deep-learning hybrid algorithm

Compressed ultrafast photography(CUP) is the fastest single-shot passive ultrafast optical imaging technique,which has shown to be a powerful tool in recording self-luminous or non-repeatable ultrafast phenomena.However, the low fidelity of image reconstruction based on the conventional augmented-Lagrangian(AL)and two-step iterative shrinkage/thresholding(Tw IST) algorithms greatly prevents practical applications of CUP, especially for those ultrafast phenomena that need high spatial resolution. Here, we develop a novel AL and deep-learning(DL) hybrid(i.e., AL+DL) algorithm to realize high-fidelity image reconstruction for CUP. The AL+DL algorithm not only optimizes the sparse domain and relevant iteration parameters via learning the dataset but also simplifies the mathematical architecture, so it greatly improves the image reconstruction accuracy. Our theoretical simulation and experimental results validate the superior performance of the AL+DL algorithm in image fidelity over conventional AL and Tw IST algorithms, where the peak signalto-noise ratio and structural similarity index can be increased at least by 4 d B(9 d B) and 0.1(0.05) for a complex(simple) dynamic scene, respectively. This study can promote the applications of CUP in related fields, and it will also enable a new strategy for recovering high-dimensional signals from low-dimensional detection.

A nonperturbative quantum electrodynamic approach to the theory of laser induced high harmonic generation

Using nonperturbative quantum electrodynamics, we develop a scattering theory for high harmonic generation （HHG）. A transition rate formula for HHG is obtained. Applying this formula, we cal- culate the spectra of high harmonics generated from different noble gases shined by strong laser light. We study the cutoff property of the spectra. The data show that the cutoff orders of high harmonics are greater than that predicted by the ＂3.17＂ cutoff law. As a numerical experiment, the data obtained from our repeated calculations support the newly derived theoretical expression of the cutoff law. The cutoff energy of high harmonics described by the new cutoff law, in terms of the ponderomotive energy Up and the ionization potential energy Ip, is 3.34Up 1.83Ip. The higher cutoff orders predicted by this theory are due to the absorption of the extra photons, which participate only the photon-mode up-conversion and do nothing in the photoionization process.

Single-shot spectral-volumetric compressed ultrafast photography

In ultrafast optical imaging,it is critical to obtain the spatial structure,temporal evolution,and spectral composition of the object with snapshots in order to better observe and understand unrepeatable or irreversible dynamic scenes.However,so far,there are no ultrafast optical imaging techniques that can simultaneously capture the spatial–temporal–spectral five-dimensional(5D)information of dynamic scenes.To break the limitation of the existing techniques in imaging dimensions,we develop a spectral-volumetric compressed ultrafast photography(SV-CUP)technique.In our SV-CUP,the spatial resolutions in the x,y and z directions are,respectively,0.39,0.35,and 3 mm with an 8.8 mm×6.3 mm field of view,the temporal frame interval is 2 ps,and the spectral frame interval is 1.72 nm.To demonstrate the excellent performance of our SV-CUP in spatial–temporal–spectral 5D imaging,we successfully measure the spectrally resolved photoluminescent dynamics of a 3D mannequin coated with CdSe quantum dots.Our SV-CUP brings unprecedented detection capabilities to dynamic scenes,which has important application prospects in fundamental research and applied science.

Controlling multiphoton excited energy transfer from Tm^(3+) to Yb^(3+) ions by a phase-shaped femtosecond laser field

The ability to control the energy transfer in rare-earth ion-doped luminescent materials is very important for various related application areas such as color display, bio-labeling, and new light sources. Here, a phase-shaped femtosecond laser field is first proposed to control the transfer of multiphoton excited energy from Tm^(3+) to Yb^(3+) ions in co-doped glass ceramics. Tm^(3+) ions are first sensitized by femtosecond laser-induced multiphoton absorption, and then a highly efficient energy transfer occurs between the highly excited state Tm^(3+) sensitizers and the ground-state Yb^(3+) activators. The laser peak intensity and polarization dependences of the laser-induced luminescence intensities are shown to serve as proof of the multiphoton excited energy transfer pathway.The efficiency of the multiphoton excited energy transfer can be efficiently enhanced or completely suppressed by optimizing the spectral phase of the femtosecond laser with a feedback control strategy based on a genetic algorithm. A(1+2) resonance-mediated three-photon excitation model is presented to explain the experimental observations. This study provides a new way to induce and control the energy transfer in rare-earth ion-doped luminescent materials, and should have a positive contribution to the development of related applications.

Engineering naphthalimide-cyanine integrated near-infrared dye into ROS-responsive nanohybrids for tumor PDT/PTT/chemotherapy

Photodynamic(PDT)and photothermal therapies(PTT)are emerging treatments for tumour ablation.Organic dyes such as porphyrin,chlorin,phthalocyanine,boron-dipyrromethene and cyanine are the clinically or preclinically used photosensitizer or photothermal agents.Development of structurally diverse near-infrared dyes with long absorption wavelength is of great significance for PDT and PTT.Herein,we report a novel near-infrared dye ML880 with naphthalimide modified cyanine skeleton.The introduction of naphthalimide moiety results in stronger electron delocalization and larger redshift in emission compared with IR820.Furthermore,ML880 is co-loaded with chemotherapeutic drug into ROS-responsive mesoporous organosilica(RMON)to construct nanomedicine NBD&ML@RMON,which exhibits remarkable tumor inhibition effects through PDT/PTT/chemotherapy in vivo.

Exploring Femtosecond Laser Ablation by Snapshot Ultrafast Imaging and Molecular Dynamics Simulation

Femtosecond laser ablation(FLA)has been playing a prominent role in precision fabrication of material because of its circumvention of thermal effect and extremely high spatial resolution.Molecular dynamics modeling,as a powerful tool to study the mechanism of femtosecond laser ablation,still lacks the connection between its simulation results and experimental observations at present.Here we combine a single-shot chirped spectral mapping ultrafast photography(CSMUP)technique in experiment and a three-dimensional two-temperature model-based molecular dynamics(3D TTM-MD)method in theory to jointly investigate the FLA process of bulky gold.Our experimental and simulated results show quite high consistency in time-resolved morphologic dynamics.According to the highly accurate simulations,the FLA process of gold at the high laser fluence is dominated by the phase explosion,which shows drastic vaporized cluster eruption and pressure dynamics,while the FLA process at the low laser fluence mainly results from the photomechanical spallation,which shows moderate temperature and pressure dynamics.This study reveals the ultrafast dynamics of gold with different ablation schemes,which has a guiding significance for the applications of FLA on various kinds of materials.